'Renting' my plane.

I have a '46 Ercoupe that can be flown under Sport Pilot rules. I have a
couple friends that are interested in 'renting' my plane. One is my cousin
who has ATP and CFI ratings and the other hasn't renewed his medical lately
so he'd like to fly it under the Sport. I have let my cousin borrow my plane
and give the other friend a checkout.

What can/should I charge my friends for borrowing my plane? Just the cost of
fuel/oil? Or should it be something like $25 dry or $50 wet (it gets about
5gph-tach time)? Both friends say they want to be able to borrow the plane
in the future. I checked my insurance policy and it says that I can allow
any current pilot to fly my plane but I don't want to get into the 'rental'
insurance costs.

I'm also considering adding Sport Pilot CFI to my private certificate and
doing some training in my Ercoupe. I have the hours in my plane and just
need to study to take the tests. This, too, would be for training my (other)
friends to fly my Ercoupe.

Or should I just forget about 'renting' it, let friends fly the plane and
eat the costs???

-Greg B.